Acoustic resonators and filters with reduced temperature coefficient of frequency

ABSTRACT

Acoustic resonator devices and filters. An acoustic resonator includes a substrate having a surface and a lithium niobate plate. A back surface of the lithium niobate plate is attached the substrate except for a portion of the lithium niobate plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on a front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm. The IDT and the lithium niobate plate configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm. Euler angles of the lithium niobate plate are [0°, β, 0°], where β is greater than or equal to 40° and less than or equal to 70°.

RELATED APPLICATION INFORMATION

This patent claim priority to provisional patent application 63/053,584, filed Jul. 18, 2020, entitled TCF OPTIMIZED XBAR DEVICES, and provisional patent application 63/088,344, filed Oct. 6, 2020, entitled OPTIMAL CUT ANGLE TO REDUCE TCF OF XBAR FILTERS, which is incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a passband or stop-band depend on the application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3^(rd) Generation Partnership Project). Radio access technology for 5^(th) generation (5G) mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5 GHz and 6 GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectional views, and a detail view of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is a schematic block diagram of a bandpass filter implemented with XBARs.

FIG. 3 is a graph of possible combinations of piezoelectric coupling and temperature coefficient of velocity for lithium niobate.

FIG. 4 is a graph of piezoelectric coupling and temperature coefficient of velocity as functions of Euler angle β for lithium niobate.

FIG. 5 is a graph of the input-output transfer function of a filter with reduced temperature coefficient of frequency.

FIG. 6 is a graph of possible combinations of piezoelectric coupling and temperature coefficient of velocity for lithium tantalate.

FIG. 7 is a graph of piezoelectric coupling and temperature coefficient of velocity as functions of Euler angle β for lithium tantalate.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional views of an XBAR 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters such as the bandpass filter 110, duplexers, and multiplexers.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having substantially parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate may be Z-cut, which is to say the Z axis is normal to the front and back surfaces 112, 114. The piezoelectric plate may be rotated Z-cut or rotated YX-cut. XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of a substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm 115 may be contiguous with the piezoelectric plate around at least 50% of the perimeter 145 of the cavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 under the diaphragm 115. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 that spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thicknesses of the IDT fingers and the piezoelectric plate in the cross-sectional views are greatly exaggerated.

Referring now to the detailed schematic cross-sectional view, a front-side dielectric layer 150 may optionally be formed on the front side of the piezoelectric plate 110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer 150 may be formed only between the IDT fingers (e.g. IDT finger 138 b) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger 138 a). The front-side dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide, alumina, or silicon nitride. A thickness of the front side dielectric layer 150 is typically less than about one-third of the thickness of the piezoelectric plate 110. The front-side dielectric layer 150 may be formed of multiple layers of two or more materials. In some applications, a back-side dielectric layer (not shown) may be formed on the back side of the piezoelectric plate 110.

The IDT fingers 138 a, 138 b may be one or more layers of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars (132, 134 in FIG. 1) of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The geometry of the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness of the piezoelectric plate 210. The width of the IDT fingers in an XBAR is not constrained to be near one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be readily fabricated using optical lithography. The thickness of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars (132, 134) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

FIG. 2 is a schematic circuit diagram and layout for a high frequency band-pass filter 200 using XBARs. The filter 200 has a conventional ladder filter architecture including three series resonators 210A, 210B, 210C and two shunt resonators 220A, 220B. The three series resonators 210A, 210B, and 210C are connected in series between a first port and a second port (hence the term “series resonator”). In FIG. 2, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 200 is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators 220A, 220B are connected from nodes between the series resonators to ground. A filter may contain additional reactive components, such as inductors, not shown in FIG. 2. All the shunt resonators and series resonators are XBARs. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all of the series resonators are connected in series between an input and an output of the filter. All of the shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators.

In the exemplary filter 200, the three series resonators 210A, B, C and the two shunt resonators 220A, B of the filter 200 are formed on a single plate 230 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 2, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 235). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity.

Each of the resonators 210A, 210B, 210C, 220A, 220B in the filter 200 has resonance where the admittance of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter 200. In over-simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter's passband and the anti-resonance frequencies of the series resonators are position above the upper edge of the passband.

The primary acoustic mode in an XBAR is a shear mode in which atomic displacements in the piezoelectric plate are lateral (i.e. parallel to the surfaces of the piezoelectric plate) but vary in a vertical direction. The direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surfaces of the piezoelectric plate.

The resonance frequency of an XBAR is proportional to the velocity of the shear primary acoustic mode in the diaphragm, and roughly inversely proportional to the diaphragm thickness. The resonance frequency of an XBAR is also dependent on the pitch and mark of the IDT fingers. In some broadband filters, a dielectric frequency setting layer, indicated by the broken rectangle 240, may be formed on the front and/or back surfaces to increase the diaphragm thickness above the thickness of the piezoelectric plate. This lowers the resonance frequencies of the shunt resonators relative to the resonance frequencies of the series resonators.

The shear wave velocity and the diaphragm thickness are both temperature dependent, with the temperature coefficient of shear wave velocity (TCV) being the dominant factor in the temperature dependence of resonance frequency.

The difference between the resonance and anti-resonance frequencies of an XBAR is determined, in part, by the electro-mechanical coupling between the electric field and the primary shear wave. This coupling depends on piezoelectric coupling coefficient e₁₅. e₁₅ is an element of a 3×6 matrix of piezoelectric coupling coefficients that describe the physical response of a piezoelectric material to an applied electric field. A larger value of e₁₅ results in more efficient coupling to the primary shear acoustic mode, which results in wider spacing between the resonance and anti-resonance frequencies of an XBAR.

FIG. 3 is a chart 300 of possible combinations of piezoelectric coupling coefficient e₁₅ and temperature coefficient of velocity TCV for all possible orientations of a lithium niobate crystal. Euler angles are a system, introduced by Swiss mathematician Leonhard Euler, to define the orientation of a body with respect to a fixed coordinate system. The orientation is defined by three successive rotations about angles α, β, and γ. The shaded area 310 contains the values of e₁₅ and TCV for all possible combinations of angles α, β, and γ. The dashed line 320 is the locus of (e15, TCV) values for lithium niobate piezoelectric plates with Euler angles (0°, β, 0°) where 0≤β≤180°.

FIG. 4 is a chart 400 of piezoelectric coupling coefficient e₁₅ and temperature coefficient of velocity TCV as functions of Euler angle β for lithium niobate with Euler angles (0°, β, 0°). Specifically, the solid curve 410 is a plot of TCV versus β. The solid curve 410 is read against the left-hand axis. TCV is expressed in part per million per degree Celsius (ppm/° C.). The dashed curve 420 is a plot of e₁₅ versus β. The dashed curve 420 is read against the right-hand axis.

Lithium niobate crystal orientations previously used for XBARs include Z-cut and rotated Y-cut. Z-cut has Euler angles=(0°, 0°, 90°). Rotated Y-cut has Euler angles=(0°, β, 0°), with β between 30 and 38 degrees. Z-cut lithium niobate has a TCV of about −102 ppm/° C. and e₁₅ of about 3.7. Rotated Y-cut lithium niobate has e₁₅ about 4.4 and TCV between about −86 ppm/° C. and −92 ppm/° C.

Inspection of FIG. 4 shows that rotated Y-cut lithium niobate with β about 67 degrees (broken line 432) has a value of e₁₅ of about 3.7 (broken line 434) which is equivalent to the e₁₅ for Z-cut lithium niobate. Rotated Y-cut lithium niobate with β about 67 degrees has a TCV of about −73 ppm/° C., which is 30% smaller (in magnitude) than the TCV of Z-cut lithium niobate. Filters comprised of XBARs using lithium niobate piezoelectric plates with β substantially equal to 67° may have performance comparable to filters using Z-cut lithium niobate with significantly less frequency dependence on temperature. In this and similar contexts, “substantially equal” means equal with defined manufacturing tolerances. The range from β=38° to β=67° offers a continuous trade-off between piezoelectric coupling and TCV. For example, a rotated Y-cut lithium niobate plate with β=60° offers 5% higher piezoelectric coupling than a plate with β=67° with only a small increase in the magnitude of TCV.

The bandwidth and other requirements of a particular filter may dictate a minimum value for e₁₅. The Euler angles (0°, β, 0°) of the piezoelectric plate may be selected with β set to the highest value in the range from 40° to 67° that provides the required minimum value of e₁₅, while minimizing, to the extent possible, the TCF of the filter.

FIG. 5 is a chart 500 of the performance of a bandpass filter implemented with XBARs formed on a lithium niobate piezoelectric plate with Euler angles (0°, 67°, 0°). Specifically, the solid line 510 is a plot of S21, the input-output transfer function, of the filter at a temperature of 25 degrees Celsius. The dashed line 520 is a plot of S21 at a temperature of 75 degrees Celsius. Both plots are results of simulations of the filter using a finite element method. The temperature coefficient of frequency (TCF) for the upper and lower band edges is about −59 ppm/° C. These TCF values are an improvement (i.e. reduction in magnitude) of 8% to 17% for the lower band edge and 13% to 24% for the upper band edge compared to prior filter designs using Z-cut lithium niobate.

FIG. 6 is a chart 600 of possible combinations of piezoelectric coupling coefficient e₁₅ and temperature coefficient of velocity TCV for all possible orientations of a lithium tantalate crystal. The shaded area 610 contains the values of e₁₅ and TCV for all possible combinations of angles α, β, and γ. The dashed line 620 is the locus of (e₁₅, TCV) values for lithium tantalate piezoelectric plates with Euler angles (0°, β, 0°) where 0≤β≤180°. FIG. 6 shows the maximum piezoelectric coupling coefficient for lithium tantalate is lower than the maximum for lithium niobate. However, lithium tantalate has a wider range of TCV values than lithium niobate. In particular, the TCV of a lithium tantalate piezoelectric plate can be selected within a wide range by selecting an appropriate value for the second Euler angle β.

FIG. 7 is a chart 700 of piezoelectric coupling coefficient e₁₅ and temperature coefficient of velocity TCV as functions of Euler angle β for lithium tantalate with Euler angles (0°, β, 0°). Specifically, the solid curve 710 is a plot of TCV versus β. The solid curve 710 is read against the left-hand axis. TCV is expressed in part per million per degree Celsius (ppm/° C.). The dashed curve 720 is a plot of e₁₅ versus temperature. The dashed curve 720 is read against the right-hand axis.

Inspection of FIG. 7 shows that rotated Y-cut lithium tantalate with Euler angles (0°, β, 0°) has a small positive TCV and e₁₅ greater than 3.0 for 0 between about 18° and 54°. For comparison, Z-cut lithium tantalate has TCV equal to −20 ppm/° C. and e₁₅ equal to about 2.65. Both TCV and e₁₅ are maximized for β between about 30° and 40°. Lithium tantalate XBARs with β greater than or equal to 30° and less than or equal to 40° can be used for moderate bandwidth filters with low temperature dependence.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic resonator device comprising: a substrate having a surface; a lithium niobate plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the lithium niobate plate forming a diaphragm that spans a cavity in the substrate; and an interdigital transducer (IDT) formed on the front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, the IDT and the lithium niobate plate configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein Euler angles of the lithium niobate plate are [0°, β, 0° ], where β is greater than or equal to 40° and less than or equal to 70°.
 2. The acoustic resonator device of claim 1, wherein β is greater than or equal to 60° and less than or equal to 70°.
 3. The acoustic resonator device of claim 1, wherein β is substantially equal to 67°.
 4. The acoustic resonator device of claim 1, wherein β is selected to set a piezoelectric coupling coefficient e₁₅ to at least a predetermined value.
 5. A filter device, comprising: a substrate; a lithium niobate plate having front and back surfaces, the back surface attached to a surface of the substrate, portions of the lithium niobate plate forming one or more diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, interleaved fingers of each of the plurality of IDTs disposed on the one or more diaphragms, wherein the lithium niobate plate and all of the IDTs are configured-such that respective radio frequency signals applied to the IDTs excite respective shear primary acoustic modes within the respective diaphragms, the lithium niobate plate comprises rotated z-cut lithium niobate, and Euler angles of the rotated z-cut lithium niobate are 0°, β, 0°, where β is greater than or equal to 40° and less than or equal to 70°.
 6. The filter device of claim 5, wherein β is greater than or equal to 60° and less than or equal to 70°.
 7. The filter device of claim 5, wherein β is substantially equal to 67°.
 8. The filter device of claim 5, wherein β is selected to set a piezoelectric coupling coefficient e₁₅ to at least a predetermined value.
 9. The filter device of claim 8, wherein the predetermined value is a minimum value of piezoelectric coupling coefficient e₁₅ necessary to satisfy a set of requirements on the filter device.
 10. The filter device of claim 5, wherein each of the plurality of IDTs is disposed on a respective diaphragm spanning a respective cavity.
 11. The filter device of claim 5, wherein the plurality of acoustic resonators are connected in a ladder filter circuit including at one or more shunt resonators and one or more series resonators.
 12. The filter device of claim 11, further comprising: a frequency-setting dielectric layer formed on one of the front surface and the back surface of the diaphragms of the one or more shunt resonators.
 13. An acoustic resonator device comprising: a substrate having a surface; a lithium tantalate plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the lithium tantalate plate forming a diaphragm that spans a cavity in the substrate; and an interdigital transducer (IDT) formed on the front surface of the lithium tantalate plate such that interleaved fingers of the IDT are disposed on the diaphragm, the IDT and the lithium tantalate plate configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein Euler angles of the lithium tantalate plate are [0°, β, 0° ], where β is greater than or equal to 18° and less than or equal to 54°.
 14. The acoustic resonator device of claim 13, wherein β is greater than or equal to 30° and less than or equal to 40°.
 15. The acoustic resonator device of claim 13, wherein the lithium tantalate plate is rotated Y-cut lithium tantalate. 